Thanks for your comments everybody. I've made a list of some Noteworthy Issues that need to be addressed after people's suggestions and playing on public servers:
-Players and props getting stuck on thin air outside the plane. Should be able to fix or at least minimise this.
-Spawn area is a bit cramped and laggy due to everybody spawning in a small room full with all of the weapons and ammo.
-I used a debug entity to add floating text instructions to the map which turns out not to work in multiplayer. Will need to substitute with custom sprites.
-Function and use of various buttons around the plane and drill objective is unclear. This should be cleared up once the new help text is added.
-It's ambiguous as to whether you can climb out of the freezer. I've never managed to but it might be possible. Not sure whether this more funny or irritating.
-Your problems go here!
how do i make the turret?
In-game? Most of them are activated in the cockpit, others can be activated by traitors or by innocents in the case of the two in the rear lower cabins. If you want to add one to a map you use the npc_turret_ceiling entity.